Gases have various
properties that we can observe with our
senses, including the gas pressure,temperature (T), mass, and the
volume (V)
that contains the gas.
Careful, scientific observation has determined that these
variables
are related to one another and that the values of
these properties determine the state of the
gas.

The relationship between temperature and volume, at a constant
number of moles and pressure, is called Charles and Gay-Lussac's
Law in honor of the two French scientists who first investigated
this relationship. Charles did the original work, which was verified
by Gay-Lussac. They observed that if the pressure is held constant,
the volume V is equal to a constant times the
temperature T:

V = constant * T

For example, suppose we have a theoretical gas confined
in a jar with a piston at the top.
The initial state of the gas has a volume qual to 4.0 cubic meters,
and the temperature is 300 Kelvin. With the
pressure and number of moles held constant, the burner has been
turned off and the gas is allowed to cool to 225 Kelvin. (In
an actual experiment, a cryogenic ice-bath would be required to
obtain these temperatures.) As the gas cools, the volume decreases to
3.0 cubic meters. The volume divided by the temperature remains a
constant (4/300 = 3/225 ).
Here is a computer animation of this process: